1. Field of Invention
The invention relates to a cylinder head gasket disposed between a cylinder block and a cylinder head of an engine.
2. Description of Related Art
In recent years, in association with improvements in the performance of motor vehicle-purpose engines and, particularly, diesel engines, the combustion pressure of the engines is increasing, and a trend toward increased in-cylinder pressures of the engines is progressing. Due to the trend toward increased in-cylinder pressures, there is a tendency toward increased relative movements between the cylinder block and the cylinder head and, therefore, increased amounts of gap between the cylinder block and the cylinder head. This leads to higher requirements for the sealing performance provided by a cylinder head gasket interposed between the cylinder block and the cylinder head.
An example of a cylinder head gasket in which the sealing performance of a cylinder periphery portion-side portion (around the combustion chambers) is intended to be improved (e.g., see Japanese Patent No. 2935544). Japanese Patent No. 2935544 discloses a cylinder head gasket in which a gasket subsidiary plate whose thickness is greater in a cylinder periphery portion-side region than in a region outward of the cylinder periphery portion-side region, and a plurality of gasket base plates each provided with a full bead formed in a cylinder periphery portion-side region are stacked. The gasket subsidiary plate is sandwiched between full-bead skirt portions of adjacent gasket base plates, and full-bead top portions of the gasket base plates sandwiching the gasket subsidiary plate are superimposed on full-bead top portions of other gasket base plates.
According to the cylinder head gasket described in Japanese Patent No. 2935544, since the thickness of the gasket subsidiary plate is greater in the cylinder periphery portion-side region than in the region outward of the cylinder periphery portion-side region, the surface pressure on the cylinder periphery portion can be increased; however, the following problems occur.
Since the cylinder periphery portion-side region of the gasket subsidiary plate and the region outward of the cylinder periphery portion-side region are provided as a single member, it is difficult in an application to a multi-cylinder engine to vary the thickness of the cylinder periphery portion-side region of the gasket subsidiary plate with respect to the individual cylinders. Thus, a problem is that it becomes difficult to change the surface pressure balance between the cylinder periphery portion-side region and the region outward of the cylinder periphery portion-side region, with respect to the individual cylinders.
Furthermore, since the full-bead top portions of the gasket base plates sandwiching the gasket subsidiary plate are stacked on full-bead top portions of other gasket base plates, that is, since adjacent gasket base plates are stacked so that the full-bead top portions of the gasket base plates face each other, there is a possibility of adjacent gasket base plates deviating from each other in a direction parallel to the base plate plane during assembly or the like. This leads to a problem that at the site of occurrence of such deviation, the tight closure characteristic declines and there occurs dispersion in the surface pressure.